1. Field
The present invention relates to disk drives for computer systems. In particular, the present invention relates to a disk drive employing offset compensation for velocity control of a voice coil motor.
2. Description of the Related Art
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
There are times when the servo control system does not have access to the embedded servo sectors yet it is still desirable to control the velocity of the actuator arm. For example, in disk drives employing ramp loading/unloading, it is desirable to control the velocity of the actuator arm so that the head is not damaged as it travels off the ramp onto the disk as well as off the disk onto the ramp. Another example is if the servo control system loses servo sector synchronization it is desirable to control the velocity of the actuator arm to facilitate re-synchronizing to the servo sectors.
Prior art techniques for controlling the velocity of the actuator arm when servo sector information is unavailable include using a voltage loop with the detected back EMF voltage generated by the VCM as the feedback. The voltage across the voice coil (the voice coil voltage) comprises a component due to the inductance L of the VCM, a component due to the resistance R of the VCM, and a component due to the velocity of the VCM referred to as the back EMF voltage. If the component due to the resistance R is canceled from the voice coil voltage, at low frequencies Ldi/dt is small leaving the back EMF voltage due to the velocity of the VCM as the dominant component.
Various analog components, such as differential operational amplifiers (op-amps) are typically employed in the feedback loop, for example to detect the voice coil voltage and/or the current flowing through a sense resistor in series with the voice coil. The various analog components may have a non-zero input offset voltage that can distort the back EMF measurement. A known technique for determining the input offset voltage involves zeroing the input to the op amp that measures the voice coil voltage and adjusting the control voltage until the detected current flowing through the sense resistor is zero. The control voltage that generates the detected zero current is then used to adjust the control voltage during normal operation in order to compensate for the offset. However, this technique may not account for the offset voltage of the current sensing op-amp, and/or it may be obfuscated by quantization error and/or signal noise that may vary depending on the ambient temperature.
There is, therefore, a need to improve the back EMF voltage measurement of a VCM in order to improve the velocity control mode of a disk drive.